Support PostgreSQL C Functions with Multiple AS Parameters

[LucaCappelletti94]

Enables parsing of PostgreSQL C language functions that specify both object file and link symbol in the AS clause.

Example

CREATE FUNCTION cas_in(input cstring) RETURNS cas
LANGUAGE c
AS 'MODULE_PATHNAME', 'cas_in_wrapper';

Fixes #2093

[iffyio]

oh, does wrapping this in a Tuple not include a parenthesis (first_expr, second_expr) when displaying the AST? wondering if/how we worked around that somehow in this case

[LucaCappelletti94]

Ah simply the following happens:

if let Some(CreateFunctionBody::AsBeforeOptions(function_body)) = &self.function_body {
        write!(f, " AS ")?;
        // Special handling for tuple expressions to format without parentheses
        // PostgreSQL C functions use: AS 'obj_file', 'link_symbol'
        // rather than: AS ('obj_file', 'link_symbol')
        if let Expr::Tuple(exprs) = function_body {
            write!(f, "{}", display_comma_separated(exprs))?;
        } else {
            write!(f, "{function_body}")?;
        }
    }